1. Field of the Invention
The present invention relates to binoculars having a focal adjustment mechanism and a dioptric deviation adjustment mechanism.
2. Description of Related Art
A known example of binoculars having a focal adjustment mechanism and a dioptric deviation (diopter difference between right and left eyes) adjustment mechanism is disclosed in Japanese Laid-Open Patent Publication No. H 3-163512. In this pair of binoculars, a focal adjustment wheel, a diopter adjustment wheel and a main shaft are provided at a bridge portion linking the left lens barrel with the right lens barrel. At the main shaft, which includes a shaft on the base side and a shaft on the front end side, the two shafts are connected with each other along the axial direction through a screw interlock. The shaft on the base side is screw-fitted with the focal adjustment wheel by a screw, whereas the shaft on the front end side is screw-fitted at the diopter adjustment wheel by a screw. Thus, the entire main shaft moves along the axial direction as the focal adjustment wheel is rotated, whereas the shaft on the front end side is driven out along the axial direction relative to the shaft on the base side as the diopter adjustment wheel is rotated. In addition, of the objective lenses provided in the left lens barrel and the right lens barrel, one is linked with the shaft on the base side and the other is linked to the shaft on the front end side. As a result, the entire main shaft moves as one along the axial direction when the user rotates the focal adjustment wheel, which allows both the left objective lens and the right objective lens to move along the optical axis to perform a focal adjustment. When the user rotates the diopter adjustment wheel, on the other hand, the shaft on the front end side of the main shaft alone is driven out along the axial direction to perform a diopter adjustment by causing the objective lens linked to the shaft on the front end side to move along the optical axis.
As described above, the binoculars disclosed in Japanese Laid-Open Patent Publication No. H 3-163512 adopt a structure that causes the entire main shaft to move for a focal adjustment and allows only the front end portion of the main shaft to be driven out for a diopter adjustment. As a result, the entire main shaft moves within the bridge during the focal adjustment, whereas the length of the main shaft increases/decreases during the diopter adjustment.
The size of the bridge where the main shaft is provided is determined in advance during the design stage by ensuring that the bridge size affords ease of use. The length of the lens barrels and the length of the bridge are both set at particularly small values when designing compact and thus portable binoculars. If the binocular structure disclosed in Japanese Laid-Open Patent Publication No. H 3-163512 is adopted in such compact binoculars, the length of the entire main shaft increases as the front end portion of the main shaft is driven out for a diopter adjustment, and since this reduces the range over which the main shaft can be moved within the bridge for a focal adjustment, the focal adjustment range, too, becomes smaller. Namely, when L represents the length along which the main shaft can move within the bridge, a relationship expressed as; L=L1+L2 is achieved by the range L1 of movement made for the focal adjustment and the range L2 of the movement made for the diopter adjustment. Thus, when the user wishes to move the objective lenses by a large extent in order to focus the binoculars on an object present within a closer range, the objective lenses cannot be moved to the focusing position if the diopter adjustment quantity is significant. This leads to a problem in that the shortest focus distance (the shortest possible distance to an object that can be in focus) cannot be reduced. There is another problem in that depending upon the specific movement restrictions that are imposed, the shortest focus distance may change in correspondence to the diopter adjustment position.
In addition, the binoculars disclosed in Japanese Laid-Open Patent Publication H3-163512 have a problem in that play readily occurs as the main shaft moves during a focal adjustment operation. As described earlier, the shaft on the base side and the shaft on the front end side constituting the main shaft are linked to each other through a screw interlock. The end of the base side shaft is supported at the focal adjustment wheel through a screw interlock as well. The end of the front end side shaft is supported at the diopter adjustment wheel at which the shaft end is fitted in a spline structure or it is supported by a movable rod (guide rod) passing through a bridge member linking the main shaft to the objective lenses. This means that the base side shaft is supported at both ends through a screw interlock. For this reason, operational play tends to occur readily during a focal adjustment. The front end side shaft, on the other hand, is supported at one end through a screw interlock and at the other end through a spline structure. While the play attributable to the use of the screw is insignificant, the shaft is allowed to deflect to the left and the right by an extent corresponding to the difference between the diameter of the through hole through which the guide rod provided at the bridge member passes and the diameter of the guide rod. Since the diameter of the through hole must be set at a fairly large value to absorb the parts tolerance with regard to the gap between the guide rod and the main shaft, operational play also occurs when the front end side shaft of the main shaft is supported by the guide rod. Such operational play at the main shaft occurring during the execution of a focal adjustment, and in particular, operational play manifesting to the left and the right of the shaft, results in an error with respect to the positions of the left and right objective lenses of the binoculars which, in turn, leads to a dioptric deviation between the left and right sides.